140 PROC. ENT. SOC. WASH., VOL. 23, NO. 6, JUNE, 1921 



another group of the sawflies, the Pamphiliinae, there exists a 

 smaller and less conspicuous, though undoubtedly identical 

 structure. No special attention has been given to this structure 

 in the Pamphiliinae, and because of this and its use in classifi- 

 cation, the organ is herewith described. It consists of a small, 

 obscure and somewhat S-shaped chitinized rod projecting from 

 a posterior median plate, which occurs in a depression on the 

 dorsad-caudad surface of the epiproct (see Plate I, Fig. A), and 

 because of the position in which it is carried by the larva the 

 postcornu can hardly be seen. (These observations were made 

 upon dead larvae and the author has not yet had an opportunity 

 to note the position or use of this structure in living larvae). 



The character of, and the position occupied by, this appendage 

 and the slight tendency for the formation of the lateral caudad 

 grooves, as indicated by the depression at the lateral margin of 

 the epiproct, seems to give ample grounds for believing it to be 

 homologous with the postcornu of the other sawflies. 



A comparative study of the postcornu of those groups ot the 

 Chalastogastra which possess this structure reveals certain 

 similarities within families and differences between them, which 

 may be described and tabulated as follows: 



1. Postcornu small, inconspicuous, projecting anteriorly, S-shaped and 



unornamented (see plate I, figs. A and E). Larvae of four genera 

 examined Pamphiliinae. 



- Postcornu large, conspicuous, projecting posteriorly and more or less 



ornamented by spines, spurs or barbs 2. 



2. Postcornu short, tube-like, circular in cross-section at apex, unornamented 



with spurs or barbs except on basal lobe (see plate I, fig. D). Larvae 

 of four genera examined ' Cephidae. 



- Postcornu long, rod-like, compressed laterally or semicircular in cross- 



section at apex, ornamented on the dorsal or ventral margin with barbs 

 or spurs 



3. Postcornu slightly curved, concave ventrally; semicircular in cross-section 



at apex, convex dorsally; ornamented on the ventral margin with spurs 

 or barbs (see plate I, fig. C). Larvae of the genus Xiphvdria ex- 

 amined .. Xiphydriidae. 



- Postcornu straight; a vertical line in cross section at apex; ornamented on 



the dorsal margin with spurs or barbs (see plate I, fig. B). Larvae of 

 five Nearctic genera examined Siricidae. 



Paired Terminal Structures 



Dr. G. C. Crampton in his excellent paper 2 on the terminal 



J " Notes on the Larvae of Some Cephidae," William Middleton, Proc. Ent. 

 Soc. Wash., vol. 19, 1917, p. 174-179. 



2 The Genitalia and Terminal Abdominal Structure of Males, and the Terminal 

 Abdominal Structure of the Larvae of "Chalastogastrous Hymenoptera" 

 Proc. Ent. Soc. Wash., vol. 21, No. 6, June, 1919, p. 137-8. 



